A spark plug is known, in which an electrode tip is joined to an electrode base for improvement in electrode durability (see, for example, Japanese Laid-Open Patent Publication No. 2010-238498). In this type of spark plug, the electrode tip is formed of a material having higher spark resistance and oxidation resistance to those of the electrode base. As such an electrode tip material, a noble metal (e.g. platinum, iridium, ruthenium or rhodium) or a material containing a noble metal can be used as a main component.
The spark plug of Japanese Laid-Open Patent Publication No. 2010-238498 includes a fused part formed by laser welding between the electrode tip and the electrode base such that the fused part has a tapered shape (so called “wedge shape”) in the emission direction of a laser beam during the laser welding.
When the spark plug is used in an internal combustion engine, thermal stress is exerted on the fused part between the electrode tip and the electrode base by combustion heat of the internal combustion engine. It is thus likely that a crack (fracture) and an oxide scale will occur at boundaries between the electrode tip and the fused part and between the electrode base and the fused part. In the case where at least one of a crack and an oxide scale is developed excessively at such boundaries, the electrode tip may become separated and fall off from the electrode base.
There is no sufficient consideration given in Japanese Laid-Open Patent Publication No. 2010-238498 to the improvement in the lifetime of the spark plug by retarding the development of a crack and an oxide scale at the boundaries between the electrode tip and the fused part and between the electrode base and the fused part.